In my U.S. Pat. No. 3,927,546, issued Oct. 15, 1974, I describe a method for the explosive forming or reforming of tubular copper molds for the continuous casting of metals, which has proved highly successful with the smaller sizes of mold. In the case of the larger sizes of mold certain problems can arise. Large molds often have substantial wall thicknesses, in a typical example approximately 4 cm. in a rectangular mold having internal cross-sectional dimensions of 25 cm. .times. 30 cm. An important advantage of the invention is that the forming process is combined with a highly advantageous hardening of the copper on the inside of the mold, but it is found that, with large molds with thick walls, the size of the explosive charge required to produce the required forming effect, and more particularly the required degree of hardening, may be such as to damage the mold and possibly the arbor on which mold is formed. In order for the required effect to be achieved, the magnitude of the shock applied by the explosive to the copper must be such as to cause it plastically to deform against the arbor so as to cause it to assume the profile of the mold and to receive the necessary work hardening. With large wall thicknesses, the necessary magnitude of shock tends only to occur in those areas where the external surface of the mold is covered by the explosive charge: this means that the charge in effect must completely jacket the mold. This not only implies a rather substantial charge, but also means that it is difficult to control the detonation pattern of the explosive so as to produce the required solely longitudinal end-to-end detonation of the charge. The large charge results in heavy stresses being applied to the arbor, whilst an incorrect detonation pattern can lead to damage to the mold itself, or failure of the mold to assume the required configuration with sufficient accuracy since the arbor becomes strained out of shape either elastically and therefore momentarily during the forming process, or permanently. Moreover, since the arbors are expensive to produce, it is undesirable that they be subject to stresses that might substantially shorten their lives. I have found that in order to obtain accurately formed molds it is important to use the smallest explosive charges that will reliably provide the required forming and hardening effect, and it is also important that detonation occurs solely longitudinally of the mold, as specified in my patent, if twisting and other deformations of the mold are not to occur.
A further problem arises in the forming of molds of large dimensions. Such molds are most conveniently and economically formed from cast sleeves. Before forming a sleeve, it is necessary to grind its inner surface to remove residual casting sand, scale and oxide and localized casting defects. This grinding may leave substantial depressions as much as 5 millimeters deep in the inner surface of the sleeve which can be difficult to remove during the forming operation and still further increase the size of the explosive charge which must be used.
Finally, with large molds there are severe practical problems in providing facilities for containing the explosion of the quite large quantities of explosives needed to provide the required forming forces.